There is a case in which it is necessary to form a thick layer on a base member to manufacture a structure. Such a structure is, for example, a combustion chamber of a space rocket engine or an aerospace rocket engine. When the combustion chamber of the rocket engine is manufactured, it is necessary, for example, to form a copper layer having a thickness of 10 mm or more on the base member formed of copper.
One method for forming such a thick metal layer is “electroplating”. However, a layer growth rate using electroplating is extremely slow. For example, it takes several months to reach a target layer thickness of about 10 mm.
To solve the above-mentioned problem, the applicant of this application proposes a technique in which the thick metal layer is deposited using a “cold spray process” in Japanese Patent Application Publication JP 2012-057203 A. The cold spray process is a process in which high-speed gas flow having a temperature lower than a melting point or softening point of material powder is formed, the material particles are supplied in the high-speed gas flow and are accelerated, and the material particles in a solid state are impinged on the base member to form the layer. The layer growth rate using this cold spray process is extremely fast compared to the case of electroplating. Therefore, it is possible to reduce a period required for manufacturing the structure by using the cold spray process.
However, unlike the case such as forming a thin oxide layer by cold spraying, when the thick layer of about 10 mm is formed by cold spraying, it is necessary to consider a following point. The point is that, when a formed layer reaches a certain degree of thickness, residual stress becomes stronger than adhesion strength and separation of the formed layer occurs. A limit whether or not the separation of the formed layer occurs is hereinafter referred to as a “separation limit”. In order to prevent the separation of the formed layer, it is necessary to remove the residual stress by heat treatment before reaching the separation limit as described in JP 2012-057203 A.
FIG. 1 schematically indicates a relationship between the thickness of the formed layer and the residual stress (internal stress). As shown in FIG. 1, in the case of forming the layer by cold spraying, the layer thickness increases with time, and accordingly, the residual stress also increases with time. If the residual stress exceeds a separation limit line, the formed layer separates. Therefore, a layer deposition process by cold spraying is stopped once before that. Then, the “heat treatment” is performed separately from the cold spray. By this heat treatment, the residual stress of the formed layer is removed. After that, the layer deposition process by cold spraying is restarted.
Thus, in order to form the thick layer of about 10 mm by cold spraying, it is necessary to repeatedly perform the layer deposition process and the heat treatment process. A unit of the processes, which is repeatedly performed, is referred to as a “unit deposition process”. The unit deposition process includes (1) a step of depositing the layer by cold spraying under conditions in which the residual stress does not exceed the separation limit line and (2) a step of performing the heat treatment so that the residual stress is removed.
As other techniques related to the cold spray, following techniques are known.
Japanese Patent No. 5,017,675 discloses a method of forming a film of about 400 μm thickness by a cold spray process. The method includes (A) a step of decreasing or removing an oxide formed on a surface of raw material powder, which is metal powder having a surface on which the oxide is formed, by hydrogen reduction treatment or acid cleaning treatment and (B) a step of colliding the raw material powder, from which the oxide has been decreased or removed, with an object to be coated by the cold spray process to form the film thereon.
Japanese Patent Application Publication JP 2010-047825 A discloses a method of forming a coating of about 1.5 mm by the cold spray process. The method includes a step of forming a metal coating on a surface of base material by projecting non-spherical heteromorphous particles made of metal onto the base material surface by the cold spray process.
The inventors of this application have recognized the following points. As described above, in order to form the thick layer using the cold spray process, it is necessary to perform the heat treatment during performing the layer deposition method. However, as the number of the heat treatment increases, layer deposition time also increases as a whole. It is not only because the heat treatment itself requires a certain amount of time, but also because it is necessary to stop a cold spray apparatus every time of heat treating and it is necessary to restart and adjust the cold spray apparatus every time of restarting the layer deposition process. That is, as the number of the heat treatment increases, layer deposition costs increase. Therefore, the fewer the number of the heat treatment (the unit deposition process), the better.